1. Field of the Invention
The present invention relates to a process for preparing an aqueous dispersion of pigment-containing polymer particles, and more specifically to a process for preparing an aqueous dispersion of pigment-containing polymer particles, which can be suitably used for a water-based ink for inkjet printing, an aqueous dispersion of pigment-containing polymer particles prepared by the above process, and a water-based ink comprising the aqueous dispersion.
2. Discussion of the Related Art
Development of a water-based ink has been desired in order to avoid the global environmental pollution caused by organic solvents. However, there are some defects in a water-based ink containing a water-soluble dye such that the ink is poor in water resistance and light fastness. Inkjet printing is carried out by directly jetting ink droplets from very fine nozzles to deposit the ink droplets on a recording medium, to form characters or images. The inkjet printing is advantageous in that its printing device produces noise at a low level. Also, the inkjet printing is excellent in operability, facilitates coloration and enables to use plain paper as a recording medium. Therefore, the inkjet printing has been widely used in these years.
In an ink used for inkjet printers, water-soluble dyes and polyhydric alcohols are used in order to prevent the ink from being clogged in the nozzles. However, this ink is poor in water resistance and light fastness. Especially, when the ink is used as an ink for thermal jet printing, there are some defects in the ink such that the dye is oxidized due to the heat of a heater, and that the ink is easily scorched on the heater surface, thereby lowering the discharging ability.
A water-based pigment ink has been proposed in order to eliminate this defect. As the water-based pigment ink, there has been proposed a pigment dispersion-type ink prepared by dispersing a pigment in water with a surfactant or a water-soluble polymer. However, there are some defects in the pigment dispersion-type ink such that the ink is insufficient in water resistance and light fastness.
Therefore, as an ink which is excellent in water resistance and light fastness, there has been proposed a pigment-containing ink prepared by incorporating a pigment into a water-insoluble polymer, and dispersing the resulting pigment-incorporated water-insoluble polymer in water.
During the preparation of the pigment-containing ink, the formation of fine particles of a pigment is generally carried out in an organic solvent or a mixed solvent of water and an organic solvent (dispersion treatment), as disclosed in, for instance, Japanese Patent Laid-Open Nos. Hei 8-183920 and Hei 8-218013. When this ink is prepared, a roll-mill, a beads-mill, a high-pressure homogenizer or the like has been used as a dispersion device.
However, there are some defects in the above process such that as materials used in the device, particularly a packing, ones which endure the organic solvent used should be selected in accordance with the kinds of the organic solvent, and that the durability of the packing for the organic solvent is generally poorer as compared to that for water.
Also, when the dispersion device is disassembled for washing, there are necessitated the prevention of evaporation of an organic solvent or collection of an organic solvent from a waste liquid obtained by washing. Moreover, there arises a defect in working environment such that the organic solvent would impart wrong influences to human bodies (for instance, skin, throat, or the like) when the organic solvent is contacted with skin or orally inhaled upon evaporation.
In addition, when a roll-mill is used, there are some defects in the roll-mill such that a treated product is dried because an organic solvent is evaporated from its roller surface, and that working environment is stained. When a beads-mill or a high-pressure homogenizer is used, there are some defects such that an entire device including a feeding tank and a manufactured product tank should be tightly sealed in order to prevent evaporation of an organic solvent contained in the treatment solution, and that an explosion-proof device should be provided for an electric motor or the like.
As described above, when an organic solvent is contained in a dispersion treatment liquid, equipments and means for handling the organic solvent are necessitated for from small-scale experimental equipments to large-scale industrial production equipments. Therefore, there has been desired the development of a water-based dispersion in which an organic solvent is not used.
Therefore, there has been proposed a process comprising dispersing a solid polymer in water, adding a pigment to the resulting dispersion and subjecting the mixture to a dispersion treatment as disclosed in Japanese Patent Laid-Open No. Hei 8-231906. However, there are some defects in this process such that the solid polymer is not dispersed in water unless it has an appropriate composition, and that heating or dispersion treatment with a stronger dispersion device is additionally necessitated in order to disperse the solid polymer in water, thereby making its industrial productivity poor.
As mentioned above, a process which does not necessitate an organic solvent at all is idealistic. However, the realization of such a process is actually difficult. Therefore, the polymer must be used in the form of a solution of an organic solvent in order to disperse the water-insoluble polymer in water, and effectively contact the water-insoluble polymer with a pigment. At a final stage, the organic solvent is removed. As mentioned above, when an organic solvent is used during the dispersion treatment, it would be difficult to deal with the organic solvent because a complicated apparatus is necessitated. Therefore, there has been desired for developing a process for preparing an aqueous dispersion not using an organic solvent in a dispersion process, which can remove the organic solvent in a process easily removing the organic solvent with a relatively convenient apparatus prior to the dispersion process.
An object of the present invention is to provide a process for efficiently preparing an aqueous dispersion of pigment-containing polymer particles without using an organic solvent in a dispersion step, giving no crookedness to a printed matter, and having excellent water resistance.
Another object of the present invention is to provide an aqueous dispersion of pigment-containing polymer particles prepared by the above process.
Still another object of the present invention is to provide a water-based ink comprising the above aqueous dispersion.
These and other objects of the present invention will be apparent from the following description.
According to the present invention, there are provided:
(1) a process for preparing an aqueous dispersion of pigment-containing polymer particles, comprising the steps of:
(A) removing an organic solvent from a mixture (I) comprising an organic solvent solution of a polymer, water and optionally a neutralizing agent to give a solvent-removed product; and
(B) subjecting the solvent-removed product obtained in step (A) to a dispersion treatment, wherein a pigment is added to the mixture (I) or the solvent-removed product;
(2) a process for preparing an aqueous dispersion of pigment-containing polymer particles, comprising the steps of:
(C) mixing an organic solvent solution of a polymer, water and optionally a neutralizing agent with each other to give a mixture (II);
(D) removing an organic solvent from the mixture (II) obtained in step (C);
(E) mixing a solvent-removed product obtained in step (D) with a pigment to give a mixture (III); and
(F) subjecting the mixture (III) obtained in step (E) to a dispersion treatment;
(3) a process for preparing an aqueous dispersion of pigment-containing polymer particles, comprising the steps of:
(G) mixing an organic solvent solution of a polymer, water, pigment and optionally a neutralizing agent with each other to give a mixture (IV);
(H) removing an organic solvent from the mixture (IV) obtained in step (G) to give a solvent-removed product; and
(I) subjecting the solvent-removed product obtained in step (H) to a dispersion treatment;
(4) a process for preparing an aqueous dispersion of pigment-containing polymer particles, comprising the steps of:
(C) mixing an organic solvent solution of a polymer, water and optionally a neutralizing agent with each other to give a mixture (II);
(K) mixing the mixture (II) obtained in step (C) with a pigment to give a mixture (VI);
(L) removing an organic solvent from the mixture (VI) obtained in step (K) to give a solvent-removed product; and
(M) subjecting the solvent-removed product obtained in step (L) to a dispersion treatment;
(5) a process for preparing an aqueous dispersion of pigment-containing polymer particles, comprising the steps of:
(N) mixing an organic solvent solution of a polymer with a pigment to give a mixture (VII);
(O) mixing the mixture (VII) obtained in step (N), water and optionally a neutralizing agent with each other to give a mixture (VIII);
(P) removing an organic solvent from the mixture (VIII) obtained in step (O) to give a solvent-removed product; and
(Q) subjecting the solvent-removed product obtained in step (P) to a dispersion treatment;
(6) an aqueous dispersion of pigment-containing polymer particles obtained by the process of any one of items (1) to (5) above; and
(7) a water-based ink composition comprising the aqueous dispersion of pigment-containing polymer particles of item (6) above.
As a polymer used for the organic solvent solution of a polymer, there can be used a water-insoluble polymer which can incorporate a pigment. Examples of the polymer include vinyl-based polymers, polyester-based polymers, polyurethane-based polymers, and the like. Among these polymers, the vinyl-based polymers are preferable. The vinyl-based polymer includes a polymer prepared by polymerizing at least one monomer selected from the group consisting of styrene, (meth)acrylic acid and (meth)acrylic acid ester. It is preferable that the weight-average molecular weight of the polymer is 3000 to 50000, from the viewpoints of increasing durability of an ink after printing and preventing the scorching on printer heads of inkjet printers in a water-based ink for inkjet printing.
It is preferable that the polymer has a salt-forming group. In this case, a neutralizing agent for neutralizing the polymer having a salt-forming group can be employed as occasion demands.
As the neutralizing agent, an acid or a base can be used in accordance with the kind of the salt-forming group. The acid includes inorganic acids such as hydrochloric acid and sulfuric acid; and organic acids such as acetic acid, propionic acid, lactic acid, succinic acid, glycolic acid, gluconic acid, glycerol carboxylic acids. The base includes tertiary amines such as trimethylamine and triethylamine; ammonia, sodium hydroxide, potassium hydroxide and the like. The amount of the neutralizing agent is not limited to specified ones. It is preferable that the amount of the neutralizing agent is adjusted so that the resulting aqueous dispersion becomes usually neutral, for instance, pH 4.5 to 9.
The organic solvent is preferably an alcohol-based solvent, a ketone-based solvent, an ether-based solvent, an aromatic hydrocarbon-based solvent, an aliphatic hydrocarbon-based solvent, or a halogenated aliphatic hydrocarbon-based solvent, and more preferably a hydrophilic organic solvent.
The alcohol-based solvent includes methanol, ethanol, isopropanol, n-butanol, tert-butanol, isobutanol, diacetone alcohol, and the like. The ketone-based solvent includes acetone, methyl ethyl ketone, diethyl ketone, methyl isobutyl ketone, and the like. The ether-based solvent includes dibutyl ether, tetrahydrofuran, dioxane, and the like. The aromatic hydrocarbon-based solvent includes benzene, toluene, and the like. The aliphatic hydrocarbon-based solvent includes heptane, hexane, cyclohexane, and the like. The halogenated aliphatic hydrocarbon-based solvent includes methylene chloride, 1,1,1-trichloroethane, chloroform, carbon tetrachloride, 1,2-dichloroethane, and the like. Among them, acetone and methyl ethyl ketone are preferable.
The concentration of the polymer in the organic solvent solution of the polymer is not limited to specified ones. It is preferable that the concentration is usually about 1 to about 60% by weight.
It is desired that the amount of water is 100 to 1000 parts by weight, preferably 200 to 500 parts by weight, based on 100 parts by weight of the organic solvent used for the organic solvent solution of the polymer to form an aqueous continuous phase.
The pigment can be any of known inorganic pigments and organic pigments. In addition, those pigments can be used together with an extender as occasion demands.
The inorganic pigment includes carbon black, metal oxides, metal sulfides, metal chlorides and the like. Among them, carbon black is preferable for a black water-based ink. The carbon black includes furnace black, thermal lamp black, acetylene black, channel black and the like.
The organic pigment includes azo pigments, diazo pigments, phthalocyanine pigments, quinacridone pigments, isoindolinone pigments, dioxazine pigments, perylene pigments, perinone pigments, thioindigo pigments, anthraquinone pigments, quinophthalone pigments and the like.
The extender includes silica, calcium carbonate, talc and the like.
It is desired that the amount of the pigment is 50 to 700 parts by weight, preferably 80 to 400 parts by weight, more preferably 80 to 200 parts by weight, based on 100 parts by weight of the solid content of the polymer, from the viewpoints of printed density and facilitation for containing the pigment in the polymer particles.
Next, each of the preparation processes will be explained below.
Process A
In Process A, first, an organic solvent is removed from a mixture (I) comprising an organic solvent solution of a polymer, water and optionally a neutralizing agent in step (A), to give a solvent-removed product.
The mixture (I) can be obtained by mixing an organic solvent solution of a polymer, water and optionally a neutralizing agent with each other with an ordinary agitator such as an anchor impeller or a turbine impeller.
The process for removing the organic solvent from the mixture (I) is not limited to specified ones. As the process for removing the organic solvent, distillation under reduced pressure is preferable, and thin film distillation under reduced pressure is more preferable. The amount of the organic solvent removed is not limited to specified ones. Usually, it is preferable that the entire amount of the organic solvent is removed from the mixture (I).
Next, the solvent-removed product is subjected to step (B). According to step (B), the solvent-removed product obtained in step (A) is subjected to dispersion treatment. During the dispersion treatment, there can be used a ball-mill, a roll-mill, a beads-mill, a high-pressure homogenizer, a high-speed agitation-type dispersion device, or the like. Among them, the high-pressure homogenizer is preferable since the amount of inorganic impurities contaminated can be reduced.
The high-pressure homogenizer includes those comprising a chamber having a fixed flow path of a treatment solution, those comprising a homogenizing valve, the width of a flow path for a treatment solution of which is adjustable, and the like. The high-pressure homogenizer comprising a chamber having a fixed flow path for a treatment solution includes Microfluidizer (trade name) commercially available from Microfluidics International Corporation; Nanomizer (trade name) commercially available from Nanomizer; Ultimizer (trade name) commercially available from Suginomachine, and the like. The high-pressure homogenizer comprising a homogenizing valve, the width of a flow path for a treatment solution of which is adjustable includes High-Pressure Homogenizer (trade name) commercially available from Rannie; High-Pressure Homogenizer (trade name) commercially available from Sanmaru Kikai Kogyo K.K.; High-Pressure Homogenizer (trade name) commercially available from Izumi Food Machinery; and the like.
The pressure during the dispersion with the high-pressure homogenizer is preferably not less than 50 MPa, more preferably not less than 80 MPa, because the polymer particles having a desired particle diameter can be readily obtained in a short period of time.
In Process A, a pigment is added to the mixture (I) or the solvent-removed product, and the resulting mixture is stirred. The stirring can be carried out by using a high-speed rotary agitation device such as a disperser.
Thus, an aqueous dispersion of pigment-containing polymer particles is obtained by Process A.
Process B
In Process B, first, an organic solvent solution of a polymer, water and optionally a neutralizing agent are mixed with each other in step (C) to give a mixture (II). The mixing method is not limited to specified ones. The mixing method can be carried out by using an ordinary agitator such as an anchor impeller or a turbine impeller; or a high-speed rotary agitation device such as a disperser.
Next, the mixture (II) is subjected to step (D). According to step (D), the organic solvent is removed from the mixture (II) obtained in step (C) to give a solvent-removed product.
The process for removing the organic solvent from the mixture (II) is not limited to specified ones. As the process for removing an organic solvent, distillation under reduced pressure is preferable, and thin film distillation under reduced pressure is more preferable.
The solvent-removed product is subjected to step (E). According to step (E), the solvent-removed product obtained in step (D) is mixed with a pigment to give a mixture (III).
The mixing of the solvent-removed product with the pigment can be carried out by using a high-speed rotary agitation device such as a disperser.
Next, the mixture (III) is subjected to step (F). According to step (F), the mixture (III) obtained in step (E) is subjected to a dispersion treatment.
This dispersion treatment can be carried out in the same manner as in step (B) of Process A.
Thus, an aqueous dispersion of pigment-containing polymer particles is obtained by Process B.
Process C
In Process C, first, an organic solvent solution of a polymer, water, a pigment and optionally a neutralizing agent are mixed with each other in step (G), to give a mixture (IV). This step (G) can be carried out in the same manner as in step (E) of Process B.
Next, the mixture (IV) obtained in step (G) is subjected to step (H).
According to step (H), the organic solvent is removed from the mixture (IV), to give a solvent-removed product. This step (H) can be carried out in the same manner as in step (D) of Process B.
The solvent-removed product obtained in step (H) is then subjected to step (I).
According to step (I), the solvent-removed product obtained in step (H) is subjected to a dispersion treatment. The dispersion treatment can be carried out in the same manner as in step (B) of Process A.
Thus, an aqueous dispersion of pigment-containing polymer particles is obtained by Process C.
Process D
In Process D, first, an organic solvent solution of a polymer, water, and optionally a neutralizing agent are mixed with each other in step (C), to give a mixture (II). This mixing can be carried out in the same manner as in step (C) of Process B.
Next, the mixture (II) is subjected to step (K). According to step (K), the mixture (II) obtained in step (C) is mixed with a pigment, to give a mixture (VI). The mixing of the mixture (II) with the pigment can be carried out in the same manner as in step (E) of Process B.
The mixture (VI) is subjected to step (L). According to step (L), the organic solvent is removed from the mixture (VI) obtained in step (K), to give a solvent-removed product. The removal of the organic solvent can be carried out in the same manner as in step (D) of Process B.
Subsequently, the solvent-removed product is subjected to step (M). According to step (M), the solvent-removed product obtained in step (L) is subjected to dispersion treatment. The dispersion treatment can be carried out in the same manner as in step (B) of Process A.
Thus, an aqueous dispersion of pigment-containing polymer particles is obtained by Process D.
Process E
In Process E, first, an organic solvent solution of a polymer is mixed with a pigment in step (N), to give a mixture (VII). This mixing of the organic solvent solution of a polymer with the pigment can be carried out in the same manner as in step (C) of Process B.
Next, the mixture (VII) is subjected to step (O). According to step (O), the mixture (VII) obtained in step (N), water, and optionally a neutralizing agent are mixed with each other, to give a mixture (VIII). This mixing can be carried out in the same manner as in step (E) of Process B.
Subsequently, the mixture (VIII) is subjected to step (P). According to step (P), the organic solvent is removed from the mixture (VIII) obtained in step (O), to give a solvent-removed product. The removal of the organic solvent can be carried out in the same manner as in step (D) of Process B.
The solvent-removed product obtained in step (P) is then subjected to step (Q). According to step (Q), the solvent-removed product is subjected to dispersion treatment. The dispersion treatment of the solvent-removed product can be carried out in the same manner as in step (B) of Process A.
Thus, an aqueous dispersion of pigment-containing polymer particles is obtained by Process E.
The average particle diameter of the polymer particles contained in the aqueous dispersion of pigment-containing polymer particles is preferably 0.01 to 0.50 xcexcm, more preferably 0.02 to 0.20 xcexcm, from the viewpoints of prevention of the generation of bleeding out of the water-based ink and dispersion stability. As occasion demands, coarse particles can be removed from the aqueous dispersion by means of centrifugation, filtration with a filter, or the like. Since the aqueous dispersion can be used as the water-based ink without any further treatments, the process for preparing an aqueous dispersion of the present invention can be a process for preparing a water-based ink comprising the aqueous dispersion of pigment-containing polymer particles.
When the aqueous dispersion of the present invention is used for a water-based ink, it is desired that the content of the pigment-containing polymer particles in the water-based ink is usually 1 to 30% by weight, preferably 2 to 15% by weight, from the viewpoints of printed density and jetting stability of a water-based ink for inkjet printing.
When the water-based ink is prepared from the aqueous dispersion of pigment-containing polymer particles of the present invention, there can be added additives such as a wetting agent, a dispersant, a defoaming agent, a mildewproof agent and a chelating agent.